1. Field of the Invention
The invention is a process and apparatus for the separation of a catalyst phase from a cracked hydrocarbon phase in the fluid catalyst cracking (FCC) of hydrocarbon. More particularly, the invention is a process and apparatus to reduce post riser cracking of cracked hydrocarbon discharged from a riser reactor. The invention is also a process and apparatus which heat integrates the riser reactor, and the catalyst regenerator, thereby reducing the heat duty in a fluid catalyze cracking (FCC) process.
2. Other Related Methods and Apparatus in the Field
Fluid catalytic cracking (FCC) processes are known in the art. State of the art commercial catalytic cracking catalysts for these processes are highly active and selective for converting hydrocarbon charge stocks to liquid fuel products. With such active catalysts it is preferable to conduct catalytic cracking reactions in a dilute phase transport type reaction system with a relatively short period of contact between the catalyst and the hydrocarbon feedstock, e.g. 0.2 to 10 seconds.
The control of short contact times, optimum for state of the art catalysts in dense phase fluidized bed reactors is not feasible. Consequently, catalytic cracking systems have been developed in which the primary cracking reaction is carried out in a transfer line reactor or riser reactor. In such systems, the catalyst is dispersed in the hydrocarbon feedstock and passed through an elongated reaction zone at relatively high velocity. In these transfer line reactor systems, feedstock acts as a carrier for the catalyst. In a typical upflow riser reactor, the hydrocarbon vapors move with sufficient velocity as to maintain the catalyst particles in suspension with a minimum of back mixing of the catalyst particles with the gaseous carrier. Thus development of improved fluid catalytic cracking catalysts has led to the development and utilization of reactors in which the reaction is carried out with the solid catalysts particles in a relatively dilute phase with the catalyst dispersed or suspended in hydrocarbon vapors undergoing reaction, e.g., cracking.
The cracking reactions are conveniently carried out in high velocity transport line reactors wherein the catalysts is moved from one vessel to another by the hydrocarbon vapors. Such reactors have become known in the art as risers or riser reactors. The catalyst and hydrocarbon mixture passes from the transfer line reactor into a first separation zone in which hydrocarbon vapors are separated from the catalyst. The catalyst particles are then passed into a second separation zone, usually a dense fluidized bed stripping zone wherein further separation of hydrocarbons from the catalyst takes place by stripping the catalyst with steam. After separation of hydrocarbons from the catalyst, the catalyst is introduced into a regeneration zone where carbonaceous residues are removed by burning with air or other oxygen-containing gas. After regeneration, hot catalyst from the regeneration zone is reintroduced into the transfer line reactor with fresh hydrocarbon feed.
As stated, state of the art catalytic cracking catalysts are highly active. With the introduction of these highly active catalysts the first separation zone has become a limiting unit operation. When catalyst is not rapidly separated from vapor and the vapor quenched once the desired reactions have taken place, the cracking reactions will continue with the production of less desirable products. Rough-cut cyclones have been used as a first separation stage between catalyst and vapor, followed by finer cut cyclones to remove fines from the vapor.
U.S. Pat. No. 4,664,888 to L. F. Castagnos, Jr. teaches a rough cut catalyst-vapor separator in a fluid catalytic cracking process. In the separator a separator surface causes the oil-catalyst mixture to undergo a 180.degree. turn. Catalyst moves toward the separator surface to form a catalytic phase. Vapor is squeezed away from the wall forming a vapor phase. A shave edge maintains the separation.
U.S. Pat. Nos. 4,764,268 and 4,624,771 both to P. A. Lane teach a fluid catalytic cracking process. A quench fluid is passed into a downstream portion of the riser reactor in the last 10 vol% to prevent overcracking of hydrocarbon products. The quench fluid is inert to cracking, e.g. water, steam or a selected hydrocarbon. The catalyst and vapor are separated after quenching. An advantageous yield of product of a desirable octane number is achieved.
Perry's Chemical Engineers'Handbook, 4th Ed., p. 18-64 teaches fan nozzles. The nozzles form a flat fan-shaped fluid sheet. The included angle of the fan is from 10 deg. to 130 deg. in standard nozzles and capacities range from 0.1 to 20 gal./minute.